1. Field of the Invention
The present invention relates to a photodetector and a production method thereof more specifically a photodetector having a sensitivity in the near-infrared region and a production method thereof.
2. Description of the Background Art
GaInNAs produced by adding nitrogen (N) to InGaAs has a band gap corresponding to the near-infrared region. Consequently, researchers and engineers have been studying to develop optical devices incorporating GaInNAs. For example, Patent literature 1 has proposed an episide-down-mounting-type photodiode that uses GaInNAs for its absorption layer and that has a laminated structure composed of an InP substrate, InP layer, GaInNAs absorption layer, InP layer, and GaInAs cap layer in this order. Nonpatent literature 1 has reported an example of a prototype production of a photodiode that uses GaInNAs(P) for its absorption layer and that has a long-wavelength-side limitation at a wavelength of 1.81 μm. The above-described prototype photodiode has a laminated structure composed of an InP substrate, buffer layer, GaInNAs(P) absorption layer, and InP cap layer in this order.
On the other hand, Patent literature 2 has disclosed the advantage of forming a GaInNAs layer through a molecular beam epitaxy (MBE) method in order to prevent the intrusion of hydrogen not only for a photodetector but also for a semiconductor laser. According to Patent literature 2, a GaInNAs layer is grown through the MBE method using a material containing no hydrogen. Then, the crystal quality is evaluated using the full width at half maximum of the intensity of photoluminescence. The evaluation confirms the improvement in the crystal quality through the use of the MBE method that uses a material containing no hydrogen. Patent literature 3 has disclosed a method to obtain a light-emitting device for emitting light in the near-infrared region. In this method, first, a GaInNAs layer is formed using a material gas containing hydrogen in a hydrogen atmosphere through an organometallic vapor phase epitaxy (OMVPE) method. Then, the layer is annealed in a nitrogen atmosphere at a temperature higher than the crystal-growing temperature. Patent literature 3 has stated that the above-described method can produce a surface-emitting laser that emits light in the near-infrared region and that has excellent crystal quality.                Patent literature 1: the published Japanese patent application Tokukaihei 9-219563        Patent literature 2: the published Japanese patent application Tokukai 2001-24282        Patent literature 3: the published Japanese patent application Tokukai 2004-165349        Nonpatent literature 1: Jian Wei et al., “Gas Source Molecular Beam Epitaxy Grown InGaAs(P)N—InP Long-Wavelength (λ>1.65 μm) Photodetectors Using a Solid Arsenic Source,” IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 13, No. 4, April 2001.        
The photodetectors having the above-described laminated structures, however, all have high dark current, thereby creating a major stumbling block for practical use. For example, according to Nonpatent literature 1, when the photodiode has an area of 6.5×10−5 cm2, a dark current of 1.2×10−8 A is produced when a voltage of −1 V is applied. It has been confirmed in Nonpatent literature 1 that the dark current cannot be suppressed even when the level of the hydrogen concentration of the GaInNAsP layer is reduced through the MBE method using a material containing no hydrogen. Accordingly, it has been presumed in the literature that the high dark current is produced not only by the formation of N—H bonding but also by another cause.